EE6351 ELECTRIC DRIVES AND CONTROL UNIT-1 INTRODUTION

EE6351 ELECTRIC DRIVES AND CONTROL UNIT-1 INTRODUTION 1. What is meant by drive and electric drive? Machines employed for motion control are called drives and may employ any one of the prime movers for supplying the mechanical energy for motion control. The drives employing electric motors as prime movers are called electric drives. 2. List out some advantages of electric drives. The advantages of electric drives are: 1. Availability of electric drives over a wide range of power a few watts to mega watts. 2. Ability to provide a wide range of torques over wide range of speeds. 3. Electric motors are available in a variety of design in order to make them compatible to any type of load. 3. List out some examples of electric drives. Examples of electric drives are, 1. Driving fans, ventilators, compressors and pumps. 2. Lifting goods by hoists and cranes. 3. Imparting motion to conveyors in factories, mines and warehouses 4. Running excavators & escalators, electric locomotives trains, cars trolley buses, lifts & drum winders etc. 4. What are the types of electric drives? The types of electric drives are, 1. Group electric drives (Shaft drive) 2. Individual Drives 3. Multi motor electric drives 5. What are the classes of duties? 1. Continuous duty 2. Short time duty operation of motor 3. Intermittent periodic duty 4. Intermittent periodic duty with starting 5. Intermittent periodic duty with starting & braking 6. Continuous duty with intermittent periodic loading 7. Continuous duty with starting & braking 8. Continuous duty with periodic load changes

6. Draw the heating and cooling curve of an electric motor. 7. What are the various function performed by an electric drive? The various function performed by an electric drive are, 1. Driving fans, ventilators, compressors & pumps etc., 2. Lifting goods by hoists & cranes 3. Imparting motion to conveyors in factories, mines & warehouses and 4. Running excavators & escalators, electric locomotives, trains, cars, trolley buses and lifts etc. 8. What is meant by four-quadrant operation? A motor operates in two modes- motoring and braking. In motoring it converts electrical energy into mechanical energy, which supports its motion. In braking it works as a generator converting mechanical energy into electrical energy, and thus opposes the motion. Motor can provide motoring and braking operations for both forward and reverse directions. 9. What is active and passive load torque? Load torque which has the potential to drive the motor under equilibrium conditions are called active load torque. Load torques which always oppose the motion and change their sign on the reversal of motion are called passive load torques. 10. What are the components of load torque? The components of load torque are, 1. Friction torque 2. Windage torque 3. Torque required doing the useful mechanical work. 11. What is meant by regenerative braking? In the regenerative braking operation, the motor operates as a generator, while it is still connected to the supply here; the motor speed is grater that the synchronous speed. Mechanical energy is converter into electrical energy, part of which is returned to the supply and rest as heat in the winding and bearing.

12. What are the different types of electric braking? The different types of electric braking are, 1. Dynamic or Rheostatic braking, 2. Counter current braking or plugging and 3. Regenerative braking UNIT II ELECTRICAL MOTOR CHARACTERISTICS 1. Why does a single phase induction motor not self-start? When a single phase supply is fed to the single phase induction motor, its stator winding produces a flux which only alternates along one space axis. It is not a synchronously revolving field, as in the case of a 2 or 3phase stator winding, fed from 2 or 3 phase supply. 2. What is meant by plugging? The plugging operation can be achieved by changing the polarity of the motor there by reversing the direction of rotation of the motor. This can be achieved in ac motors by changing the phase sequence and in dc motors by changing the polarity. 3. Give some applications of DC motor. Shunt: driving constant speed, lathes, centrifugal pumps, machine tools, blowers and fans, reciprocating pumps Series: electric locomotives, rapid transit systems, trolley cars, cranes and hoists, conveyors Compound: elevators, air compressors, rolling mills, heavy planners. 4. What are the different types of electric braking? The different types of electric braking are, 1. Dynamic or Rheostatic braking, 2. Counter current or plugging and 3. Regenerative braking 5. When does an induction motor behave to run off as a generator? When the rotor of an induction motor runs faster than the stator field, the slip becomes negative. Regenerative braking occurs and the K.E. of the rotating parts is return back to the supply as electrical energy and thus the machine generates power. 6. Define Slip Slip is defined as the percentage difference between the synchronous speed and the actual speed. S = N s N r N s where, N s = synchronous speed in rpm. N r = rotor speed in rpm S = Slip

7. What is back emf in DC motor? As the motor armature rotates, the system of conductor come across alternate north and south pole magnetic fields causing an emf induced in the conductors. The direction of the emf induced in the conductor is in opposite to current. As this emf always opposes the flow of current in motor operation it is called as back emf. 8. Draw the speed torque characteristics of DC shunt motor. 9. Draw the speed torque characteristics of DC series motor. 10. What is meant by regenerative braking? In regenerative braking operation motor operate as generator, while it still connected to supply. Here the motor speed greater than the base speed and back emf greater than applied voltage. Mechanical energy converted into electrical energy, part of which is returned to the supply and rest of the energy is last as heat in the winding and bearings. 11. What is meant by dynamic braking? During dynamic braking the kinetic energy of the motor is converted into electric energy. This energy is dissipated in resistive elements.

UNIT-III STARTING METHODS 1. List out the Starters used to start a DC motor. The starters used to start a DC motor are: 1. Two point Starter 2. Three point Starter 3. Four point Starter 2. List out the Starters used to start an Induction motor. 1. D.O.L Starter (Direct Online Starter) 2. Star-Delta Starter 3. Auto Transformer Starter 4. Reactance or Resistance starter 5. Stator Rotor Starter (Rotor Resistance Starter) 3. What are the protective devices in a DC/AC motor starter. 1. Over load Release (O.L.R) or No volt coil 2. Hold on Coil 3. Thermal Relays 4. Fuses (Starting /Running) 5. Over load relay 4. Why are motor take heavy current at starting? When 3 phase supply is given to the stator of an induction motor, magnetic field rotating in space at synchronous speed is produced. This magnetic field is cut by the rotor conductors, which are short circuited. This gives to induced current in them. Since rotor of an induction motor behaves as a short circuited secondary of a transformer whose primary is stator winding, heavy rotor current will require corresponding heavy stator balancing currents. Thus motor draws heavy current at starting 5. What are the methods to reduce the magnitude of rotor current (rotor induced current) at starting?. 1.By increasing the resistance in the rotor circuit 2.By reducing the magnitude of rotating magnetic field i.e by reducing the applied voltage to the stator windings. 6. What is the objective of rotor resistance starter (stator rotor starter)? To include resistance in the rotor circuit there by reducing the induced rotor current at starting. This can be implemented only on a slip ring induction motor.

7. Why are squirrel cage induction motors not used for loads requiring high starting torque? Squirrel cage motors are started only by reduced voltage starting methods which lead to the development of low starting torque at starting. This is the reason why squirrel cage induction motors are not used for loads requiring high starting torque. 8. Give the relation between line voltage and phase voltage in a (i) Delta connected network (ii) Star connected network Delta connected network: V phase = V line Star connected network: V phase = V line / 3 9. List out some advantages and disadvantages of D.O.L starter. Advantages: 1. Highest starting torque 2. Low cost 3. Greatest simplicity Disadvantages: 1. The inrush current of large motors may cause excessive voltage drop in the weak power system 2. The torque may be limited to protect certain types of loads. 10. Draw the speed-torque characteristics of an Induction motor with various values of rotor resistance. Rotor Resistance Increasing T max Torque Speed 11. What is the function of no-voltage release coil in DC motor starter? As long as the supply voltage is on healthy condition the current through the NVR coil produce enough magnetic force of attraction and retain the starter handle in ON position against spring force. When the supply voltage fails or becomes lower than a prescribed value then electromagnet may not have enough force to retain so handle will come back to OFF position due to spring force automatically.

UNIT-IV CONVENTIONAL AND SOLID STATE SPEED CONTROL OF DC DRIVES 1. Write the expression for speed for a DC motor. Speed N = k (V-I a R a ) where V = Terminal Voltage in volts Ia = Armature current in Amps Ra = Armature resistance in ohms = flux per pole. 2. What are the ways of speed control in dc motors? 1. Field control -by varying the flux per pole -for above rated speed 2. Armature control -by varying the terminal voltage -for below rated speed 3. List out the limitations of field control. 1. Speed lower than the rated speed cannot be obtained. 2. It can cope with constant kw drives only. 3. This control is not suitable to application needing speed reversal. 4. What are the ways of field control in DC series motor? 1. Field diverter control 2. Armature diverter control 3. Motor diverter control 4. Field coil taps control 5. Series-parallel control 5. What are the main applications of Ward-Leonard system? 1. It is used for colliery winders. 2. Electric excavators 3. In elevators 4. Main drives in steel mills and blooming and paper mills. 6. List out the merits and demerits of ward-leonard method of speed control? Merits 1. Full forward and reverse speed can be achieved 2. A wide range of speed control is possible 3. Power is automatically regenerated to the AC line through the MG set when speed is reduced. 4. Short time over load capacity is large 5. The armature current of the motor is smooth

Demerits 1. High initial cost 2. The overall efficiency is low, less than 80% because of an additional MG set. 3. Costly foundation and a large amount of space is required 4. This drive produces noise 5. It require frequent maintenance. 7. What are the merits and demerits of rheostatic control method? 1. Impossible to keep the speed constant on rapidly changing loads. 2. A large amount of power is wasted in the controller resistance. 3. Loss of power is directly proportional to the reduction in speed. Hence efficiency is decreased. 4. Maximum power developed is diminished in the same ratio as speed. 5. It needs expensive arrangements for dissipation of heat produced in the controller resistance. 6. It gives speed below normal, not above. 8. What are the advantages of field control method? 1. More economical, more efficient and convenient. 2. It can give speeds above normal speed. 9. What are the different types of chopper? 1. First quadrant chopper-motoring control 2. Second quadrant chopper-braking control 3. Two quadrant chopper 4. Four quadrant chopper 10. What is the principle of the field control method of speed control of DC shunt motors? The speed of the DC motor can be controlled by varying the field flux. This method of speed control can be used for increasing the speed of the motor above its rated speed, because the speed of the motor is inversely proportional to the field flux. 11. What are the advantages of thyristor control on speed control of Dc motor? 1. Basic operation is simple and reliable 2. Time response is faster 3. Operating efficiency is high, about 95% 4. Small size 5. Less weight 6. Low initial cost 12. List out advantages of DC six pulse converter compared with three pulse converter. 1. Current should be continuous 2. Require less filter circuit 3. It gives two quadrant operation

13. Write the applications of DC chopper. 1. Electric traction 2. Steel mills 3. Printing mills 4. Textile mills 5. Paper mills 6. Hoists 7. Machine tools 8. Cranes and excavators UNIT V CONVENTIONAL AND SOLID STATE SPEED CONTROL OF A.C. DRIVES 1. What is a controlled rectifier? A controlled rectifier is a device which is used for converting controlled dc power from a control voltage ac supply. 2. Write some applications of phase control converters. Phase control converters are used in the speed control of fractional kw dc motors as well as in large motors employed in variable speed reversing drives for rolling mills. with motors ratings as large as several MW s. 3. What is the main purpose of freewheeling diode? Freewheeling diode is connected across the motor terminal to allow for the dissipation of energy stored in motor inductance and to provide for continuity of motor current when the thyristors are blocked. 4. What is a full converter? A full converter is a tow quadrant converter in which the voltage polarity of the output can reverse, but the current remains unidirectional because of unidirectional thyristors. 5. What is natural or line commutation? The commutation which occurs without any action of external force is called natural or line commutation. 6. What is forced commutation? The commutation process which takes place by the action of an external force is called forced commutation. 7. What is a chopper? A chopper is essentially an electronic switch that turns on the fixed-voltage dc source for short time intervals and applies the source potential to motor terminals in series of pulses.

8. What is voltage commutation? A charged capacitor momentarily reverse-bias the conducting thyristor to turn it off. This is known as voltage commutation. 9. What is current commutation? A current pulse is forced in the reverse direction through the conducting thyristor. As the net current becomes zero, the thyristor is turned OFF. This is known as current commutation. 10. What is load commutation? The load current flowing through the thyristor either becomes zero (as in natural or line commutation employed in converters) or is transferred to another device from the conducting thyristor. This is known as load commutation. 11. What are the different means of controlling induction motor? 1. Stator voltage control. 2. Frequency control 3. Pole changing control. 4. Slip power recovery control. 12. What are the two ways of controlling the RMS value of stator voltage? 1. Phase control 2. Integral cycle control